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Tuesday
Sep242013

The future is uncertain

Image: Repsol, SEG. Click for the abstract.

SEG Day 2. In the session entitled Exploration and Uncertainty Analysis, I was underwhelmed with the few talks that I attended, except for the last one of the session entitled, Measuring time-map uncertainty

Static uncertainty

It is commonly uttered that different data processing companies will produce different results; seismic processing is non-unique, and so on. But rarely do I get to see real examples of the kind of variances that can occur. Bruce Blake from Repsol showed seismic imaging results that came back from a number of contractors. The results were truly shocking. The example he showed was an extreme case of uncertainty caused by inadequate static solutions caused by the large sand dunes in Libya. The key point for me is exemplified by the figure shown on the right: the image from one vendor suggests a syncline, the image from the other suggest an anticline. Beware!

A hole in the theory

In the borehole sonic session, Xinding Fang, a student from MIT, reinforced a subtle but profound idea: it is tricky to measure the speed of sound in a rock when you drill a hole into it. The hole changes the stress field, and induces an anisotropic stiffness around the circumference of the borehole where sonic tools make their measurements. And since waves take the shortest travel path from source to receiver, speeds that are measured in the presence of an artificial stress are wrong.

Image: Xindang Fang, SEG. Click for the abstract.

The bigger issue here that Xinding has elucidated is that we routinely use sonic logs to make time-depth relationships and tie wells, especially in the absence of a check-shot survey. If it works, it works, but if ever discrepancies exists between seismic and well, the interpreter applies a stretch or a squeeze without much thought. Some may blame the discrepancy on dispersion alone, but that's evidently too narrow. Indeed, we rarely bother to investigate the reasons.

There's a profound point here. We have to drop the assumption that logs are the 'geological' truth upon which to hang an interpretation. We have to realize that the act of making the measurement changes the very thing we want to measure. 

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Reader Comments (7)

It seems like you should be able to correct the speeds for the presence of the hole — in the elastic case, the stress fields are well known analytically (they are used in fracture mechanics around crack tips, for instance) and speed of sound can be found from
c^2 = d(pressure)/d(density) holding entropy constant.

The pressure should be 1/3 (sigma_11 + sigma_22 + sigma_33) if sigma is the stress tensor.

If you carry out that analysis, that will let you relate the measured sound speed to the sound speed "at infinity" which is what you're interested in.

September 24, 2013 | Unregistered Commenterparticle_person

Just as I always thought - the only true measurement is seismic, provided you can process it properly!

September 25, 2013 | Unregistered CommenterDuncan Emsley

@Duncan: Yes! Well, you can choose between a poorly-constrained measurement of an unperturbed earth, and a well-constrained measurement of a highly perturbed one. Take your pick :)

PS I hope life is good back in Blighty!

September 25, 2013 | Registered CommenterMatt Hall

Yeah, The Repsol should probably be up there for best talk.

The other cool one was Alison something from BP. She showed basically this. http://i.imgur.com/bmIzPRA.png

September 26, 2013 | Unregistered CommenterToastar

@Toastar: I don't know what this says about me as a person, but I got goosebumps when I saw that picture. Amazing.

Here's the paper for anyone who's interested: Investigation of near-vertical fluid escape feature above the Frampton anticline in the south central Gulf of Mexico utilizing improved seismic imaging and 3D geobody extraction, Alison Henning, Francis Rollins, and Ryan Martin. Awesome.

September 27, 2013 | Registered CommenterMatt Hall

@ Evan
In my short adventure in Norway I've come to really appreciate how good it is to have a checkshot survey for correcting sonic logs and achieving a solid well tie. Many a geophysicist I've talked to over there would tie a well [edit: with] checkshot for interpretation but exclude it in the depth conversion.

October 2, 2013 | Unregistered CommenterMatteo

@Matteo,
Imagine, just for a second, an onshore world where every well had a checkshot. What if it became part of the drilling operation? Like a fish finder for the drill bit.

Can you clarify the last sentence of your comment? A well without a checkshot would still be tied, but not used as a control point in depth conversion? Isn't that a non sequitur? I mean, if you've tied events between time and depth, you now, by definition, have velocities for depth conversion.

October 3, 2013 | Registered CommenterEvan Bianco

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